System and method for monitoring the topography of a wafer surface during lithographic processing

ABSTRACT

A system for monitoring wafer surface topography during a lithographic process is described that includes projection optics that illuminate a portion of the wafer surface. The system further includes at least one off-axis wafer surface gauge that monitors wafer surface height relative to the projection optics as well as at least one backplane gauge that monitors wafer position relative to a backplane. The system also includes a filter that translates time-domain measurements of off-axis wafer surface gauge and the backplane gauge into space-domain measurements. A coordinate transformer is included that transforms the space-domain measurements into a single coordinate system. A computational element that combines the space-domain measurements with a focus set-point to determine correction data is also included together with a delay line for storing the correction data until the wafer has moved a predetermined distance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional patent application of U.S. patentapplication Ser. No. 09/638,902, filed Aug. 15, 2000, now U.S. Pat. No.6,633,050, and hereby incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to lithographic processing. Moreparticularly, this invention relates to a system and method formonitoring the topography of a wafer surface during lithographicprocessing.

2. Background Art

Lithography is a process used to create features on the surface ofsubstrates. Such substrates can include those used in the manufacture offlat panel displays, circuit boards, various integrated circuits, andthe like. A frequently used substrate for such applications is asemiconductor wafer. While this description is written in terms of asemiconductor wafer for illustrative purposes, one skilled in therelevant art would recognize that other substrates could be used withoutdeparting from the scope of the instant invention.

During lithography, a wafer is disposed on a wafer stage and held inplace by a chuck. The chuck is typically a vacuum or electrostatic chuckcapable of securely holding the wafer in place. The wafer is exposed toan image projected onto its surface by exposure optics located within alithography apparatus. While exposure optics are used in the case ofphotolithography, a different type of exposure apparatus may be useddepending on the particular application. For example, x-ray, ion,electron, or photon lithographies each may require a different exposureapparatus, as is known to those skilled in the relevant art. Theparticular example of photolithography is discussed here forillustrative purposes only.

The projected image produces changes in the characteristics of a layer,for example photoresist, deposited on the surface of the wafer. Thesechanges correspond to the features projected onto the wafer duringexposure. Subsequent to exposure, the layer can be etched to produce apatterned layer. The pattern corresponds to those features projectedonto the wafer during exposure. This patterned layer is then used toremove exposed portions of underlying structural layers within thewafer, such as conductive, semiconductive, or insulative layers. Thisprocess is then repeated, together with other steps, until the desiredfeatures have been formed on the surface, or in various layers, of thewafer.

Step-and-scan technology works in conjunction with a projection opticssystem that has a narrow imaging slot. Rather than expose the entirewafer at one time, individual fields are scanned onto the wafer one at atime. This is done by moving the wafer and reticle simultaneously suchthat the imaging slot is moved across the field during the scan. Thewafer stage must then be asynchronously stepped between field exposuresto allow multiple copies of the reticle pattern to be exposed over thewafer surface. In this manner, the sharpness of the image projected ontothe wafer is maximized.

While using a step-and-scan technique generally assists in improvingoverall image sharpness, image distortions generally occur in suchsystems due to imperfections within the projection optics system,illumination system, and the particular reticle being used. Such imagedistortions are frequently due to the poor focus that results from thewafer surface being located somewhere other than in the desired focalplane of the projection optics. Since the surfaces of wafers are seldomplanar, especially after multiple processing steps, focus problems areoften related to the inability to know precisely how far the wafersurface is from the projection optics along the illumination axis of thelithography apparatus. This, in turn, often stems from the fact thatmost typical sensors or gauges used to measure the separation betweenthe projection optics and the wafer surface cannot be located along theaxis of illumination.

What is needed is a system and method that can be used to determine theseparation between a wafer surface and the projection optics along theaxis of illumination so that accurate focus can be maintained.

BRIEF SUMMARY OF THE INVENTION

A system for monitoring wafer surface topography during a lithographicprocess is described, including projection optics to illuminate aportion of the wafer surface. The system further includes at least oneoff-axis wafer surface gauge that monitors wafer surface height relativeto the projection optics, as well as at least one backplane gauge thatmonitors wafer position relative to a backplane. The system alsoincludes a filter that translates time-domain measurements of off-axiswafer surface gauge and the backplane gauge into space-domainmeasurements. A coordinate transformer is included that transforms thespace-domain measurements into a single coordinate system. Acomputational element that combines the space-domain measurements with afocus set-point to determine correction data is also included togetherwith a delay line for storing the correction data until the wafer hasmoved a predetermined distance.

In a system according to the present invention, at least one off-axiswafer surface gauge is located a first distance from said axis ofillumination, the first distance being substantially equal to thepredetermined distance.

A system according to the present invention can also include at leastone stalk gauge that monitors the distance between the projection opticsand the backplane. Additional such stalk gauges can be included.

In a system according to the present invention, the filter can be afinite-impulse-response filter having a width that is controllable basedon a velocity associated with movement of the wafer relative to theprojection optics. Furthermore, the finite-impulse-response filter canoutput the space-domain measurements in response to a spatial interrupttriggered by a space clock determiner. The space clock determiner cantrigger the spatial interrupt when the wafer moves a predetermineddistance along a predetermined axis of movement. The predetermineddistance can be between about 0.1 and 1.0 millimeters, and preferablyapproximately one-half a millimeter, and the predetermined axis ofmovement can be approximately parallel to the surface of the wafer.

Also disclosed is a method of monitoring wafer surface topography duringa lithographic process. Such a method includes a first step of capturingwafer position and surface data at a first time when the wafer is at afirst wafer location. Next is a step of generating correction data for asecond wafer location prior to the wafer reaching the second waferlocation. After this correction data is generated, it is stored in aspatial delay line. Finally, in a method according to the presentinvention, the wafer is moved based on the correction data when thewafer is at said second wafer location at a second time.

A method according to the present invention can further include a stepof capturing backplane position data with a plurality of stalk gauges,as well as a step of converting the wafer position and surface datacaptured from a time-domain into a space-domain.

A method according to the present invention can also include a step oftransforming at least some of the data captured from a first coordinatesystem into a second coordinate system such that all of the datacaptured is associated with a single coordinate system. Once this isdone, a step of combining the wafer surface data and the wafer positiondata with focus set-point data in order to produce the correction datacan be performed.

Also disclosed is a virtual gauging system for use in a lithographicprocess that includes means for gauging a region at a surface of a waferwhen the region is located away from an axis of illumination producingwafer surface data, while other portions of the wafer are beingilluminated. Further included in such a system is means for convertingthe wafer surface data into wafer correction data. Also included ismeans for adjusting a separation distance between an exposure lens andthe region at the surface of the wafer based on the correction data whenthe region is located at the axis of illumination.

In the system so described, the means for gauging includes at least twowafer surface gauges located on opposite sides of an illumination slot.The means for converting the wafer surface data into wafer correctiondata includes means for determining a direction of travel of the wafer,the wafer correction data being based on data produced by one of the twowafer surface gauges located on the side of the illumination slot thatcorresponds to the direction of travel of the wafer. The means forconverting the wafer surface data into wafer correction data can includea finite-impulse-response filter that can be triggered by a spatialinterrupt and has a width that can be modified in response to thevelocity of travel of the wafer.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 is an illustration of a lithography apparatus that can be used inconnection with the present invention.

FIG. 2 is an illustration of wafer surface gauge and illumination slotlayout within a lithography apparatus like that of FIG. 1.

FIG. 3A is an illustration of a particular wafer surface and wafersurface gauge situation along the line A—A in FIG. 2.

FIG. 3B is another illustration of another particular wafer surface andwafer surface gauge situation also along the line A—A in FIG. 2.

FIG. 4A is an illustration of a wafer surface being illuminated within alithography apparatus at a first time.

FIG. 4B is an illustration of the wafer surface shown in FIG. 4A beingilluminated at a subsequent time.

FIG. 5 is an illustration of a system according to the presentinvention.

FIG. 6A is a detailed illustration of a first portion of a systemaccording to the present invention.

FIG. 6B is a detailed illustration of a second portion of the system ofFIG. 6A according to the present invention.

FIG. 7 is a diagram of a coordinate system that can be used inconnection with the present invention.

FIG. 8 is an illustration of a method of virtual gauging according tothe present invention.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The drawings in which an elementfirst appears is indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION OF THE INVENTION

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those skilled inthe art with access to the teachings provided herein will recognizeadditional modifications, applications, and embodiments within the scopethereof and additional fields in which the present invention would be ofsignificant utility.

FIG. 1 illustrates a lithography apparatus 100. Lithography apparatus100 includes projection optics 105. Projection optics 105 can includethe elements necessary to project an image of a reticle (not shown) heldby a reticle stage 106 onto a wafer 110. Wafer 110 can be asemiconductor wafer, or any other substrate that can be lithographicallyprocessed. Wafer 110 is held in place by a chuck (not shown) on a waferstage 115. The wafer stage 115 is disposed above a backplate 120 havingan upper surface 121 that is a backplane, which serves as a positionalreference that can be used by backplane servos (not shown) for movingthe wafer stage 115 relative to the backplate 120. This movement can bemonitored by backplane gauges 125, 126.

Distances between the projection optics 105 and the wafer 110 can bemonitored by wafer surface gauges 130, 131. These wafer surface gauges130, 131 are located on either side of an exposure lens 140. Duringlithographic processing, accurate knowledge of the distance betweenexposure lens 140 and wafer 110 along the axis of exposure (i.e.,on-axis) 141 is desirable. However, on-axis measurements are difficult.The arrangement shown instead accomplishes off-axis measurements becausethe wafer surface gauges 130, 131 are located adjacent to, but off, theexposure axis. It should be noted, as would be apparent to one skilledin the relevant art, that an actual exposure lens has been depicted forillustration purposes to aid in the description of lithography apparatus100, but the actual arrangement of the lens can differ from that shownwithout departing from the scope of the present invention.

Not shown but included within the structure of FIG. 1, are the necessaryservos that effectuate wafer stage movement. As discussed above, theseservos include one or more backplane servos that cause movement of thesurface of the wafer relative to the exposure lens 140. Also includedwithin the structure of FIG. 1 are stalk gauges 145 and 146. These stalkgauges monitor any movements of the backplate 120 relative to theprojection optics 105. An example of these elements can be found in theMicrascan III tool, previously manufactured by Silicon Valley Group,Inc., 901 Ethan Allen Hwy, Ridgefield, Conn. 06877 (now ASML US Inc.).

FIG. 2 illustrates an exemplary arrangement of wafer surface gauges130–133 in relation to an exposure slot 200. The exposure slot is theregion in which the wafer is actually being exposed (or illuminated) atany given moment during a lithographic process. Wafer gauges 130–133 aresituated about this slot in order to determine how far the wafer is fromthe exposure lens during processing. Off-axis measurements taken by thewafer gauges 130–133 can simply be averaged together to approximateon-axis distances. This technique introduces errors as discussed belowin connection with FIG. 3, which illustrates a typical spatialarrangement taken along the cross-section A—A depicted in FIG. 2.

In FIG. 3A, an exemplary non-planar wafer surface 310 is depictedtogether with wafer surface gauges 130, 131 and exposure lens 141. Dueto the measurements taken by wafer surface gauges 130 and 131, thedesired focal plane has been determined to be along dotted line 330.This makes sense to the system since both surface gauges “see” the wafersurface at dotted line 330, and so the system assumes that theseoff-axis measurements correspond to the on-axis dimension. However, dueto the contour at the actual wafer surface 310, the on-axis dimensiondiffers significantly from the off-axis measurements. Thus, the systemwill have a focus set for dotted line 330 instead for wafer surface 310in the arrangement depicted in FIG. 3A.

FIG. 3B is an illustration of an arrangement similar to that shown inFIG. 3A. In the arrangement of FIG. 3B, wafer surface 320 exhibits anincrease in height between the wafer surface gauges 130, 131. Thus, asin the arrangement of FIG. 3A, the situation shown in FIG. 3B results ina focus setting that assumes a wafer surface at 320, rather than at 330.

In both of the situations shown in FIGS. 3A and 3B, the desired focalplane has been indicated at dotted line 330. The determination of wherethe desired focal plane is set is based on the particular tool as wellas the particular environment in which the particular tool is beingused. The determination of the precise location of the desired focalplane is beyond the scope of this invention. Methods for suchdetermination are known to those skilled in the art, and so will not bedescribed in more detail herein.

As used herein, the term “focus set-point” means the dimension betweenthe exposure lens and an on-axis point on the surface of the wafer whenthat point is at the desired focal plane. This dimension is alsoreferred to by those skilled in the relevant art as the “separationdistance.”

Thus, as has been described above, the use of wafer surface gauges oneither side of the axis of exposure is insufficient to accuratelydetermine the appropriate focus setting on the axis of exposure. Thepresent invention overcomes the shortcomings discussed above bycoordinating measurements taken by the wafer surface gauges, movementsof the wafer stage, and open loop corrections, in order to move thesurface of the wafer at the axis of exposure to the desired focal plane.The present invention accomplishes this by making open loop moves basedon information derived from off-axis sensors and placing a wafer in thecorrect focal plane in real time. Data on future exposure targets areacquired while ones under the lens are being exposed. A spatial delayline is employed, translating timed measurements to spatial ones inorder to operate a widely varying scan speeds. A simple, one-dimensionalversion of this technique will be described below in connection withFIGS. 4A and 4B.

FIG. 4A is an illustration of a wafer 401 with surface profile 402 beingilluminated by lens 405. Included are a backplane 415 as well as wafersurface gauges 411 and 410. In FIG. 4A, wafer surface gauge 411 is shownas a dotted line because the present invention relies only on the use ofone wafer surface gauge, depending on the direction of travel of thewafer. The desired focal plane is shown in FIG. 4A as dotted line 420.

FIG. 4A illustrates the situation at a first time T1. As seen in FIG.4A, at time T1, the wafer is positioned correctly such that the surfaceof the wafer lies within the focal plane 420. Meanwhile, wafer surfacegauge 410 is disposed above wafer position a. At time T1, the distancebetween wafer surface gauge 410 and the surface of the wafer 402 atlocation a is Ws1. Also at time T1, backplane gauges (not shown) measurethe distance between the backplane 415 and a structure holding the wafer(not shown) as Bp1. By subtracting the focus set-point from Ws1, acorrection dimension, C1, can be determined. Thus, at time T1, locationa at the wafer surface 402 is a distance C1 too far from the desiredfocal plane 420, while the surface of the wafer directly below theexposure lens is within focal plane 420.

FIG. 4B illustrates the structure of FIG. 4A at a second time T2 afteran open loop correction has been made. At time T2, the wafer has beenmoved up away from the back plane 415 such that the separation of theback plane and the wafer is now equal to Bp1+C1. In this way, wafersurface location a is now within focal plane 420. This adjustment isreferred to as an open loop adjustment, because the system does not takeany wafer surface measurements relevant to position a at time T2.Instead, at time T2, wafer surface gauge 410 measures a second dimensionat another wafer surface location, that dimension being indicated asWsi. This dimension will be used in a similar manner as to that whichdimension Ws1 was used in order to make the open loop correction shownin FIG. 4B.

Thus, as has been shown in connection with FIGS. 4A and 4B, the presentinvention uses open loop corrections that are correlated with wafermovement in order to place the wafer surface location being illuminatedinto the desired focal plane. While FIGS. 4A and 4B illustrate a simple,one-dimensional system, the present invention is adaptable to a fullrange of motion, for example in six degrees of freedom.

The present inventor has discovered that by converting time-domainmeasurements into space-domain measurements, the appropriate correctionscan be made. This will be discussed more fully below in connection withFIG. 5.

FIG. 5 is an illustration of a high-level block diagram of a virtualgauging system according to the present invention. A lithographyapparatus 501 like that shown in FIG. 1 is included in the system.Lithography apparatus 501 includes wafer surface gauges 502, back planegauges 503, and stalk gauges 504. As with the lithography apparatus 100of FIG. 1, the actual number of gauges used within the lithographyapparatus 501 illustrated in FIG. 5 is not limited by the particularstructure shown. For example, while only two stalk gauges 504 areillustrated in FIG. 5 (as well as in FIG. 1), the number of stalk gaugesmay vary. For example, in a preferred embodiment of the presentinvention, three stalk gauges are included so that the backplanelocation relative to the projection optics can be determined accurately.Likewise, while only two wafer surface gauges 502 are shown, more can beused, as would be apparent to one skilled in the relevant art given thisdescription.

The outputs of the various gauges used are each input into an associatedfinite-impulse-response (FIR) filter. Specifically, outputs from wafersurface gauges 502 are input into FIR filter 505, outputs from backplanegauges 503 are input to FIR filter 506, and outputs from stalk gauges504 are input into FIR filter 507.

The time-domain measurements coming from the various gauges areconverted into space-domain measurements. In other words, instead ofmeasurements being related to stage locations at a snapshot in time,measurements coming out of the various FIR filters are instead relatedto predetermined spatial movements of the wafer stage. Thus, instead ofhaving measurements corresponding to every half second in time, themeasurements out of the FIR filters can correspond to everyhalf-millimeter of stage movement, for example. Also included within thestructure of FIG. 5 is an input for the current x,y position of thewafer stage 508. The current x,y position of the wafer stage 508 isdetermined from mechanisms that drive the wafer stage along withadditional sensors associated with those mechanisms, not shown in thefigures. Such sensors determine the x,y position of the wafer stage withinterferometers similar to those used for the gauges described inconnection with FIG. 1.

Additionally input into the system of FIG. 5 is the focus set-point 509.As discussed above, the focus set-point 509 is the dimension at whichthe surface of the wafer needs to be separated from the exposure lens inorder to put the wafer surface into the desired focal plane.

In order to add and/or subtract various dimensions together in themanner described, for example, in connection with FIGS. 4A and 4B, alldimensions need to be characterized in terms of a single coordinatesystem. Thus, boxes labeled coordinate transformer, 510, 511 and 512,are used to transform the various dimensions received into a singlecoordinate system, for example the wafer surface gauge positions. Thus,the output from the wafer surface position FIR filter 505 does not needto be transformed, while the output from the back plane position FIRfilter 506 as well as the stalk position FIR filter 507 both need to betransformed into the coordinates of the wafer surface positions. In thisway, all of the dimensions can be expressed simply in terms of the wafersurface position. Following the computation symbols (each of which canbe implemented as a computational element) included within FIG. 5, itcan be seen that the output from the wafer surface position FIR filter505 is subtracted from the desired focus set-point 509, as indicated atcomputational element 513. Meanwhile, the output of stalk position FIRfilter 507, after being transformed at 511, is subtracted from theoutput from the backplane FIR filter 506, after being transformed attransformer 510. This is indicated at computational element 514. Thissum is then added to the focus set-point and wafer stage positioncomputation that was performed at 513. The sum calculated atcomputational element 515 in the system shown in FIG. 5 will next bedescribed in terms of the structure shown in FIGS. 4A and 4B. Thecalculation of the dimension C1 in FIG. 4A is determined atcomputational element 513 in FIG. 5, since at computational element 513,the wafer surface position is subtracted from the focus set-point. Thiscomputation is analogous in FIG. 4A to determining the differencebetween Ws1 and the focus set-point. Then, in order to figure out thedimensions shown in FIG. 4B, Bp1+C1, the dimension C1, which wascalculated at computational element 513 in FIG. 5, must then be added tothe dimension Bp1, which is essentially the current location of the backplane in terms of the wafer surface. Thus, the computation at point 515in FIG. 5 is simply the addition of the backplane location with thedesired offset to correct for the wafer surface irregularity at a firstpoint.

As can be seen in FIG. 5, also included is the computation of a stalkposition sum at computational element 514. The sum calculated at 514 isused to add any variation in position between the backplane and theprojection optics as determined by the stalk gauges.

Delay line 516 thus has as inputs the desired backplane location inorder to put the wafer surface at the proper focus set point, as well asthe current x,y position associated with the particular place on thewafer surface corresponding to the desired backplane location. Delayline 516 will thus include a number of x,y positions as well asnecessary backplane dimensions associated with those positions.

It should be noted that the delay line is a collection of correctiondata that can be characterized as having a spatial, rather thantemporal, delay. Thus, if the delay line represents, for example, a 9.5mm delay, and if the wafer surface gauge is 9.5 mm away from the axis ofexposure, then the output of the delay line will be the necessarybackplane location at the current exposure axis.

The output of delay line 516 is combined with the current stalk gaugereadings at computational element 517. Computation 517 is includedwithin the system shown in FIG. 5 because the location of the backplanerelative to the projection optics may have changed during the previous9.5 mm (for example) movement of the wafer stage. This real-time stalkcompensation computation 517 is then used within a coordinatetransformer 518 along with the original output of delay line 516 inorder to drive a backplane servo 519.

Backplane servo 519 moves the wafer stage such that the backplane is thedesired distance from the rear of the wafer stage, as determined fromthe output of the delay line 516. In this way, again returning to FIGS.4A and 4B, the movement between the arrangement shown in FIG. 4A and thearrangement shown in FIG. 4B can be accomplished. While FIG. 5 hasillustrated a high-level view of the present invention, FIGS. 6A and 6Billustrate a more detailed view.

FIG. 6A illustrates details of a portion of a system according to thepresent invention, with FIG. 6B illustrating the remainder of thesystem. System 600 includes wafer surface gauges 601. These wafersurface gauges 601 are analogous to the wafer surface gauges 130, 131 asshown in FIG. 1. Wafer surface gauges 601 can be alternating-current(“AC”) capacitance gauges familiar to those skilled in the relevant art.The precise number of wafer surface gauges is not critical to thepresent invention and could be determined by one skilled in the relevantart. Typically, the output from wafer surface gauges 601 will be a 16kHz AC signal.

System 600 also includes backplane gauges 602 as well as stalk gauges603. As with the wafer surface gauges 601, backplane gauges 602 andstalk gauges 603 can be AC capacitance gauges. The output from the stalkgauges 603 is also typically a 16 kHz AC signal, while the output fromthe backplane gauges 602 is typically an 8 kHz AC signal. As with thewafer surface gauges 601, the precise number and operation of thebackplane gauges 602, and the stalk gauges 603, can be determined by oneskilled in the relevant art and is not critical to this invention.

The output from each of the set of gauges is subject to signalconditioning. For example, wafer surface gauges 601 are subject tosignal conditioning 605. Signal conditioning 605 includes the componentsnecessary to translate the 16 kHz AC signal into a one (1) kHz digitalsignal. Thus, signal conditioning 605 can include the necessaryelectronics to convert the AC data signal into digital positionalinformation. Such digital information can be a 16 bit digital signalrepresentative of the output from the wafer surface gauges 601. Thisdigital signal can then be stepped down to a 1 kHz digital signal with a500 Hz low pass finite impulse response decimation filter. Thus, theoutput from signal conditioning 605 will include Z elevational data aswell as Tx and Ty data at one kHz. This data will be explained furtherin connection with FIG. 7.

FIG. 7 is an illustration of a coordinate system including dimensionswithin a lithographic system. The structure illustrated in FIG. 7includes a wafer 700 as well as a coordinate system overlying that wafer701. As depicted, an illumination slot 702 is being scanned across thewafer in an x direction. Also shown are wafer surface gauges 703 alongthe side of illumination slot 702. It should be noted that while eightwafer gauges are shown in FIG. 7, other numbers of gauges, for examplefour wafer gauges, can be used without departing from the scope of thepresent invention. As can be seen from coordinate system 701, the Zdimension is the dimension of elevation off the top surface of the wafertowards the illumination slot 702. Meanwhile, Ty is the rotationaldimension about the Y axis, while Tx is the rotational dimension aboutthe X axis. By characterizing the surface of the wafer in terms of the Zdimension as well as Ty and Tx, the system can determine the separationbetween the wafer surface and the exposure lense at any given point.

Thus, returning to FIG. 6A, the wafer surface gauges 601, backplanegauges 602, and stalk gauges 603 are all used to determine a Z dimensionas well as, in the case of the wafer surface gauges 601 and thebackplane gauges 602, Tx and Ty rotational dimensions.

Backplane gauges 602 are subject to signal conditioning 606, similar tothe signal conditioning 605 conducted on the output of the wafer surfacegauges 601. As with signal conditioning 605, signal conditioning 606results in backplane data including Z data Tx data and Ty data at 1 kHz.

Stalk gauges 603 are also subject to signal conditioning 607. However,rather than producing Tx and Ty data for stalk gauges 603, signalconditioning includes the necessary components to output a single Zdimension at 1 kHz for the stalk gauges. Thus, the output from signalconditioning 607 is a 1 kHz signal representing the separation betweenthe backplane and the projection optics along the exposure axis.

Also input to system 600 is x, y stage position 604. As described abovein connection with FIG. 5, the x, y stage position 604 is the current x,y position of the wafer stage as monitored by sensors associated withthe wafer stage servos. While x, y stage position 604 is shown as anadditional input with two dimensions, this input could include anadditional value corresponding to the reticle x dimension.Alternatively, this input could simply include one dimension such as thex dimension. The x, y stage position input 604 is used to monitor stagetravel along a scanning axis. Thus, any desirable location informationcan be used. Whatever stage position information is used will ultimatelydrive the spatial interrupt that translates the timed measurements intospatially separated measurements. Thus, if the primary scan direction ofthe system 600 is along the x axis, then it is only necessary that thestage position be monitored in the x dimension.

The output from x, y stage position 604 is subject to signalconditioning 608. Signal conditioning 608 can correspond to the signalconditioning 605 through 607 discussed above. Signal conditioning 608results in an output corresponding to stage position, for example an xposition and a y position, at 1 kHz. Also output from signalconditioning 608 is a time signal corresponding to the output positionalsignal. The output from signal conditioning 608 is used both to drive aspatial interrupt as well as to control a wafer gauge valid generator609 and a gauge switcher 610.

Because the system 600 according to the present invention needs to useonly a leading gauge during scanning operation, it is necessary for thesystem to know which gauge is the leading gauge. Referring to FIGS. 4Aand 4B, the leading gauge in the system illustrated in those figures isgauge 410. Thus, the output from gauge 411 is irrelevant. This isbecause the scan direction in the structure shown in FIGS. 4A and 4B isto the right and therefore gauge 410 is the leading gauge. Wafer gaugevalid generator 609 determines from the stage position output fromsignal conditioning 608 in which direction the stage is moving, and thuswhich gauges are the leading gauges. Furthermore, at various timeswithin the lithographic process, the wafer surface gauges are turned offin order to avoid saturation. This is accomplished by gauge switcher610. Thus, outputs from gauge switcher 610 are fed back to signalconditioning 605 to turn off wafer surface gauges 601. Moreover, outputfrom gauge switcher 610 is also fed into master spatial FIR filter 611.The output from gauge switcher 610 into master spatial FIR filter 611includes a signal having one bit for each wafer surface gauge. The bitscan be changed to indicate whether or not the data from a particularwafer surface gauge is valid.

Output from signal conditioning 608 corresponding to stage location isalso fed into a spatial FIR filter 612. Spatial FIR filter 612 is usedto constantly monitor stage location. This stage location is constantlyfed to space clock determiner 613. Space clock determiner 613 usesoutput from spatial FIR filter 612 in order to translate movement of thestage into spatial interrupts 614. Thus, spatial FIR filter 612 andspace clock determiner 613 work in conjunction to produce a spatialinterrupt 614 upon regular movements of the stage. For example, in atypical system, spatial interrupts are desired with every one-halfmillimeter of movement of the wafer stage. Since the spatial FIR filter612 is continuously monitoring stage position and feeding that positioninto space clock determiner 613, space clock determiner 613 waits untilthe wafer stage has moved one-half a millimeter since the last time aspatial interrupt 614 was fired. Once the wafer stage travels anadditional, for example, one-half millimeter, the space clock determinercauses another spatial interrupt to fire. Thus, 614 represents an inputto master spatial FIR filter 611 that includes a spatial interrupt thatis fired at every one-half millimeter of movement of the wafer stage.These interrupts then control the output of master spatial FIR filter611 that will be discussed with more detail below in connection withFIG. 6B.

Velocity information 615 is fed into FIR generator 616 and then intomaster spatial FIR filter 611. This velocity input can be used by masterspatial FIR filter 611 to accommodate various speeds of wafer stagetravel. However, velocity input into master spatial FIR filter 611 isnot necessary for the present invention. The operation of master spatialFIR filter 611 will now be described.

Master spatial FIR filter 611 is a finite-impulse-response filter havinga width adjusted for stage speed to match the physical width of theexposure lens. Signals fed into master spatial FIR filter 611 from thewafer surface gauges 601, backplane gauges 602, stalk gauges 603, and x,y stage position 604 are buffered within master spatial FIR filter 611.A positional signal is chosen as a driver, for example the x stageposition. The velocity input to master spatial FIR filter 611 from FIRgenerator 616 is used to control the width of the master spatial FIRfilter 611. When an interrupt is received from the space clockdeterminer 613 at 614, master spatial FIR filter 611 uses velocity datafrom FIR generator 616 to output positional information.

Positional information output from master spatial FIR filter 611includes stage position 617. Stage position 617 can include an xcoordinate, y coordinate, and any other stage positional coordinatedesired, as discussed above. Also output from master spatial FIR filter611 is backplane information 618. Backplane information 618 includes Zaxis data as well as Tx and Ty data. Also output from master spatial FIRfilter 611 is stalk data 619. Stalk data 619 includes Z axis data.Finally, output from master spatial FIR filter 611, is wafer surfacedata 620. Wafer surface data 620 includes Z axis data for each wafersurface gauge. Thus, the label Wij is meant to include a Z axis datavalue for each such wafer surface gauge.

The portion of the system 600 shown in FIG. 6A thus represents theinputs to master spatial FIR filter 611, the operation of master spatialFIR filter 611, as well as the outputs from that filter. Briefly stated,whenever a spatial interrupt is received from space clock determiner 613at 614, master spatial FIR filter 611 outputs the current stage locationas well as the backplane location, the stalk dimensions, and the wafersurface gauge dimensions. Thus, master spatial FIR filter 611 serves totranslate data from the time domain to data in the space domain. Thedata output from master spatial FIR filter 611 can then be used by otherportions of system 600, as will be described below in connection withFIG. 6B.

FIG. 6B illustrates additional components within system 600.Specifically, FIG. 6B illustrates those components within system 600 onthe output side of master spatial FIR filter 611. Thus, inputs 617, 618,619 and 620 in FIG. 6B correspond to like numbered outputs from FIG. 6A.

In the structure shown in FIG. 6B, stage position data 617 and backplaneposition data 618 are input into a backplane to wafer gauge coordinatetransformer 621. Coordinate transformer 621 transforms backplanecoordinates into corresponding left and right wafer gauge coordinates.Thus, the output from coordinate transformer 621 corresponds to thebackplane information in terms of both wafer gauge left and wafer gaugeright positional data.

Stalk data 619 is then subtracted from the output of coordinatetransform 621 at a computational element 622. Meanwhile, wafer surfacegauge data 620 is transformed into wafer gauge positional data at 623.This transformation involves taking the various wafer gauge data pointscorresponding to the various wafer gauges and producing both left andright wafer gauge data sets including Z axis data as well as Tx data forall wafer gauge sensors that are valid. Again, wafer gauge validity isdetermined as described elsewhere herein in connection with wafer gaugevalid generator 609 of FIG. 6A. The positional information output from623 is then subtracted from focus set point 624 at computational element625. Thus, the output from computation 625 includes wafer gaugecorrections for both left and right wafer gauges. In other words, dataoutput from computation 625 includes a left Z correction and a left Txcorrection, as well as a right Z correction and a right Tx correction.The output from computation 625 is then added to the output frompreviously described computation 622 at computational element 626.

The output from computational element 626 thus includes both right dataand left data. Both the right data and the left data include Zcorrection data Tx correction data, x data, y data, as well as otherdata as can be determined by one skilled in the relevant art given thisdescription.

A switch 627 is then used to select between left and right data based onscan direction sensing 628. Scan direction sensing 628 is simply anelement that uses, for example, stage positional information todetermine in which direction the stage is traveling and sets switch 627accordingly. Again, the present invention relies on only data collectedfrom the leading gauges, and so it is necessary to determine whichgauges are the leading gauges.

Output from switch 627 is fed into the gauge to slot spatial delay line629. Gauge to slot spatial delay line, or the spatial delay line, 629 issimply a continuous loop, or buffer, of data that stores correctionalinformation received from switch 627. Thus, entries within spatial delayline 629 include stage position information together with correctioninformation. Spatial delay line 629 delays outputting information fromswitch 627 until the stage has moved to the desired location. Forexample, assume a situation where correction information has beengenerated at positions 100 mm, 101 mm, 102 mm, etc. and inserted intodelay line 629. Further assume that the leading gauge to lens distanceis 9.5 mm. When the stage is at 91.5 mm, the delay line will output datadetermined at stage position 101, since 91.5+9.5 equals 101. For stagepositions in between the fixed delay lines positions, the commands canadopt a straight line interpolation.

Proper output is derived from the delay line 629 in the followingfashion. The present stage position, adjusted for the gauge to lensdistance, is used as a basis for searching the delay line 629 for theappropriate correction to be sent. This is done by examination of thestage position information in the delay line 629 which represents thestage position reference used when the correction was computed.

Spatial delay line 629 includes as an additional input delay lineinitialization 630. The purpose of delay line initialization 630 is tofill spatial delay line 629 with data upon start-up of the system. Delayline initialization 630 includes an input 631 from space clockdeterminer 613, discussed above in connection with FIG. 6A. Input signal631 is a signal that indicates that the space clock determiner 613 hassent its first interrupt to master spatial FIR filter 611. The occasionof the first interrupt thus marks the start of the system. Delay lineinitialization 630 also receives as inputs current stage position 632and backplane position 633. Stage position signal 632 comes from theoutput of signal conditioning 608, discussed above in connection withFIG. 6A, while backplane position 633 comes from the output of signalconditioning 606, also described above in connection with FIG. 6A. Withthese inputs, delay line initialization 630 fills spatial delay line 629with data. This data is merely space filler data used within spatialdelay line 629 to prevent erroneous corrections. Thus, the specificdelay line initialization inputs into spatial delay line 629 are notcritical and could be determined by one skilled in the relevant artgiven this disclosure.

Output from spatial delay line 629 thus includes current correctiondata. This data represents the current wafer correction necessary toplace the surface of the wafer within the desired focal plane. In orderto accommodate any structural changes that may have taken place betweenthe time at which the data was initially captured and the time that haspassed since, real-time stalk correction takes place at 634. Input 635into real-time stalk correction at 634 is taken from the output ofsignal conditioning 607. Thus, any time variations in the distancebetween the projection optics and the backplane as measured by the stalkgauges is factored in at 634 by adding the data as indicated in FIG. 6B.

After computation 634, correction data includes the following: Zcoordinate, Tx coordinate, Ty coordinate, and stage position (x, y).This correction data is then fed into wafer gauge to backplanecoordinate transformer 636. Coordinate transformer 636 produces asoutputs backplane correction information including Z data, Tx data, andTy data, all in terms of backplane location. This data is then fed intobackplane command limiter 637.

Backplane command limiter 637 is a safety feature that preventscollisions within the system 600. Backplane command limiter 637 includesthe limitations on backplane movement and prevents any correctioncommand from exceeding those backplane limitations. Thus, backplanecommand limiter 637 is not necessary, but is preferred.

Next, the correction data is sent to backplane servo 638. Backplaneservo 638 includes the necessary drive and compensation componentsfamiliar to those skilled in the relevant art. Backplane servo 638produces within it the necessary signals to drive servo motors that movethe wafer into the correct position as indicated by the correction data.For example, servo 638 includes feedback indicating current backplaneposition in order to move the wafer to the desired location. Currentbackplane position is taken from the output of signal conditioning 606,described above in connection with FIG. 6A. By subtracting currentbackplane position information from the correction position information,a correction command results within the servo. Thus, if the correctiondata is the same as the current backplane position, no correctioncommand results.

A system as described herein can be used in a method of virtual gauginglike that shown in FIG. 8. FIG. 8 illustrates a method of virtualgauging 800 in accordance with the present invention. In a first step810, wafer position and surface data are captured at a first time, Tiwhen the wafer is at a first wafer location, al.

In a next step 820, correction data is generated for a second waferlocation prior to the wafer reaching that location. This step involvesinputting the data collected in step 810, after being appropriatelymodified, into a spatial FIR filter like that described in connectionwith the embodiments above, for example a spatial FIR filter like masterFIR filter 611 described in connection with FIG. 6A. This data is outputfrom the spatial FIR filter in response to spatial interrupts generatedin the manner described elsewhere herein. After being output, the datais then subjected to coordinate modification such that all the data iswithin the same coordinate system. Once all the data has been put intothe same coordinate system, it can then be combined together in themanner discussed above in connection with the system of the presentinvention. Once the correction data has been so generated, it is storedin a spatial delay line, in a next step 830.

In a final step 840, the wafers move based on correction data when thewafer is in the second location a2 at a second time T2. It should benoted in connection with this final step that it is not necessary thatthe wafer already be in the final location before it is moved inresponse to the correction data. In other words, the wafer stage can bemoving the wafer into the next position while it is at the same timeresponding to the correction data corresponding to that next positionthat has been previously stored in the spatial delay line.

A system and method like that discussed herein can be implemented withhardware, software, or firmware using components known to those skilledin the relevant art(s). For example, a processor within a generalpurpose computer can be used to implement a system and to perform amethod like that disclosed herein, with respect to those elements (e.g.,computational elements, coordinate transformers, etc.) that are nototherwise defined herein.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. For example, while the invention hasbeen described in terms of a wafer, one skilled in the art wouldrecognize that the instant invention could be applied to any type ofsubstrate used in a lithography process. It will be understood by thoseskilled in the art that various changes in form and details can be madetherein without departing from the spirit and scope of the invention asdefined in the appended claims. Thus, the breadth and scope of thepresent invention should not be limited by any of the above-describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents.

1. A system for monitoring wafer surface topography during alithographic process comprising: projection optics that illuminate aportion of the wafer surface, said projection optics having an axis ofillumination; at least one off-axis wafer surface gauge that monitorswafer surface height relative to said projection optics; at least onebackplane gauge that monitors wafer position relative to a backplane; afilter that translates time-domain measurements of said at least oneoff-axis wafer surface gauge and said at least one backplane gauge intospace-domain measurements; a coordinate transformer that transforms saidspace-domain measurements into a single coordinate system; acomputational element that combines said space-domain measurements witha focus set-point to determine correction data; and a delay line forstoring the correction data until the wafer has moved a firstpredetermined distance.
 2. The system of claim 1, wherein said at leastone off-axis wafer surface gauge is located a first distance from saidaxis of illumination, and wherein said first distance is substantiallyequal to said first predetermined distance.
 3. The system of claim 1,further comprising at least one stalk gauge that monitors the distancebetween said projection optics and said backplane.
 4. The system ofclaim 3, wherein said at least one stalk gauge comprises three stalkgauges.
 5. The system of claim 1, wherein said filter is afinite-impulse-response filter.
 6. The system of claim 5, wherein saidfinite-impulse-response filter has a width that is controllable based ona velocity associated with movement of the wafer relative to saidprojection optics.
 7. The system of claim 5, wherein saidfinite-impulse-response filter outputs said space-domain measurements inresponse to a spatial interrupt triggered by a space clock determiner.8. The system of claim 7, wherein said space clock determiner triggerssaid spatial interrupt when said wafer moves a second predetermineddistance along a predetermined axis of movement.
 9. The system of claim8, wherein said predetermined axis of movement is approximately parallelto the surface of the wafer.
 10. The system of claim 8, wherein saidsecond predetermined distance is between about 0.1 and 1.0 millimeters.11. The system of claim 10, wherein said second predetermined distanceis equal to about one-half of a millimeter.
 12. A method of monitoringwafer surface topography during a lithographic process comprising thesteps of: (a) capturing wafer position and surface data at a first timewhen the wafer is at a first wafer location; wherein said capturingwafer position and surface data step includes capturing backbonesposition data; (b) generating correction data for a second waferlocation prior to the wafer reaching the second wafer location; (c)storing the correction data in a spatial delay line; and (d) moving thewafer based on the correction data when the wafer is at said secondwafer location at a second time.
 13. The method of claim 12, whereinsaid step (a) includes capturing backplane position data with aplurality of stalk gauges.
 14. A method of monitoring wafer surfacetopography during a lithographic process comprising the steps of: (a)capturing wafer position and surface data at a first time when the waferis at a first wafer location; (b) generating correction data for asecond wafer location prior to the wafer reaching the second waferlocation, wherein the generating step includes converting the waferposition and surface data captured during said step (a) from atime-domain into a space-domain and transforming at least some of thedata captured during said step (a) from a first coordinate system into asecond coordinate system such that all of the data captured during saidstep (a) is associated with a single coordinate system; (c) storing thecorrection data in a spatial delay line; and (d) moving the wafer basedon the correction data when the wafer is at said second wafer locationat a second time.
 15. The method of claim 14, further comprising thestep of combining the wafer surface data and the wafer position datawith focus set-point data in order to produce the correction data.